The present invention relates to a thermal printing system and a thermal printing head apparatus of the dot print type in which alphanumeric characters, symbols, etc. are printed in the form of combinations of dots.
A conventional thermal printing system with a thermal printing head having a series of heating elements Y1-Y7 arrayed vertically is shown in FIG. 1. For executing thermal printing by means of this printing system, the printing system successively moves the printing head at fixed intervals on a recording paper in the direction orthogonal to the array of the heating elements, that is, in a horizontal direction when the array direction of the heating elements Y1-Y7 is vertical, while at the same time selectively energizing the heating elements Y1-Y7 with a pattern of the symbol to be printed, for example, a character A. Through this movement of the thermal head, the character A denoted as 201 in FIG. 1 is depicted on a matrix (X1-X5).times.(Y1-Y7) on the recording paper. Throughout this printing of the character A, the heating element Y4 is energized to allow printing at all the dot locations, X1-X5, in the row. More specifically, this element is alternately placed in a dot formation mode for heating and then in a space mode for cooling. When printing is performed at high speed, the heat generated in the proceeding dot formation mode is insufficiently cooled in a space mode following the dot formation mode. Therefore, in the succeeding dot formation mode, the heating element is overheated due to the remaining heat. This causes the printed dot to be larger in size or thicker in graduation than that of the previous dot formation mode, deformed in its configuration. This leads to inaccurate printing, and characters that are difficult or impossible to read.
To avoid such problems, the conventional thermal printing system changes drive conditions for the thermal printing head depending on whether the dot formation mode is repeated successively or not. Generally, the thermal printing head is heated and cooled with the application of a high level control pulse and a low level control pulse, respectively. When the dot formation mode is not repeated successively, the pulse width is properly selected for setting the heating time. When the dot formation mode is repeated successively, the pulse width of the first control pulse is selected to be wider than the pulse width of the succeeding control pulses. The reason for this follows. As previously mentioned, at the time of high speed printing, before the heat generated in the first dot formation mode is properly dissipated, the next dot formation mode starts. In the second dot formation mode, the temperature of the heating element at the start of the mode is higher than that in the first dot formation mode. To eliminate this temperature difference, the pulse widths of the second and succeeding dot formation modes are set to be narrower than that in the first dot formation mode. By selecting the pulse widths of the control pulses successively applied to the heating element, the heating times in the second and subsequent dot formation modes are shortened, so that the peak heating temperature is equal to that of the first dot formation mode. In this way, the printed dots obtained in the successive dot formation modes are printed uniformly. The change of the drive condition for the heating elements overworks the controller and deteriorates the operating efficiency of the thermal printing system.
Additionally, because of the thermal operation, the thermal printing system is susceptible to a change in ambient temperature. To solve the temperature change problem, a temperature compensating circuit has been incorporated into the thermal printing system.
Use of the temperature compensating circuit, however, makes the system arrangement complicated and increases the cost of the overall system.